Fluorescent lamp containing amalgam-forming material for reducing stabilization time

ABSTRACT

A very high output fluorescent lamp having an electrode mount structure sealed in each end which includes a cathode coil supported on a pair of lead wires. At least one of the mount structures further includes a heat shield plate attached thereto and supported transversely in the tubular lamp envelope between the cathode coil and its respective end of the lamp. A quantity of amalgam-forming material, such as indium, is coated on the side of the heat shield plate facing away from the cathode coil and toward the end of the lamp for absorbing mercury when the lamp is turned off, holding the mercury on the heat shield while the lamp is inoperative, and, when the lamp is ignited, quickly releasing the mercury into the relatively cool end chamber of the lamp.

United States Patent 119 Johnson et al.

1 1 FLUORESCENT LAMP CONTAINING AMALGAM-FORMING MATERIAL FOR REDUCING STABILIZATION TIME [75] Inventors: Walter A. Johnson, Beverly;

Howard W. Milke, Danvers, both of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

1221 Filed: Apr. 22, 1974 121 1 Appl. No.: 463,004

[ Aug.5, 1975 3,619,697 11/1971 Evans 313/174 Primary ExaminerPalmer C. Demeo Attorney, Agent, or FirmEdward J. Coleman 571 I ABSTRACT A very high output fluorescent lamp having an electrode mount structure sealed in each end which includes a cathode coil supported on a pair of lead wires. At least one of the mount structures further includes a heat shield plate attached thereto and supported transversely in the tubular lamp envelope between the cathode coil and its respective end of the lamp. A quantity of amalgam-forming material, such as indium, is coated on the side of the heat shield plate facing away from the cathode coil and toward the end of the lamp for absorbing mercury when the lamp is turned off, holding the mercury on the heat shield while the lamp is inoperative, and, when the lamp is ignited, quickly releasing the mercury into the relatively cool end chamber of the lamp.

9 Claims, 3 Drawing Figures PATENTEU 5M5 3,898,531 1 RELATIVE LIGHT OUTPUT 1 FLUORESCENT LAMP CONTAINING AMALGAM-FORMING MATERIAL FOR REDUCING STABILIZATION TIME BACKGROUND OF THE INVENTION This invention relates to low-pressure mercury vapor discharge lamps and, more particularly, to very high output (VHO) fluorescent lamps employing one or more heat shields for creating a cool region at one or both ends of the lamp.

It is well known that the light output of a fluorescent lamp is a function of the mercury vapor pressure, which in turn often depends upon the temperature of the coldest region of the glass envelope of the lamp. It is further known that the envelope cold spot temperature for most efficient lamp operation is approximately 40C, which causes a mercury vapor pressure of approximately 4 to 6 X 10 Torr to occur inside the lamp. Often, due to high lamp loading or high ambient temperatures, the envelope temperatures and mercury vapor pressure rise above the optimum value.

Conventional T12 1.5 inches diameter) fluorescent lamps are designed to operate at a power loading of approximately l0 watts per foot of lamp length and reach peak light output at an ambient temperature of approximately 25C when operated at this loading In an effort to provide more light with a smaller number of lamps and'fixtures, so-called very high output (VHO) fluorescent lamps have been developed and are presently being marketed, particularly for reprographic applications, such as use in the light compartment of photocopying apparatus. These lamps (when made in the Tl 2 size) operate at loadings of approximately 25 watts per foot. Since the mercury vapor pressure increases as the loading and the operating temperatures of the lamp increase, means to compensate for this condition in such highly-loaded lamps must be provided to prevent excessive vapor pressure and a resulting drop in light output and efficiency. One method commonly used for providing such compensation in VHO lamps is to employ one or two heat shields in the lamp. Each heat shield is customarily attached to the mount structure and located between the electrode and the end of the envelope at one end or both ends of the fluorescent lamp. The function of the heat shield is to provide cool end chambers in the lamp for maintaining the condensed pool of excess mercury at the proper temperature. This, in turn, controls the mercury vapor pressure and keeps it within the aformentioned range for optimum efficiency and light output. Thus heat shields are generally located in VHO lamps in order to provide a light output level which would not be possible if the central bulb-temperature controlled the mercury vapor pressure.

A difficulty created by the use of heat shields, however, is the long time required for stabilization of light output in a new lamp. That is, upon installation. it requires a long time for the mercury to migrate to the cool zone provided. For example, it is common for VHO fluorescent lamps operating at 1500 ma. to require from o to 24 hours to stabilize. Subsequently, if such a lamp is not moved and not subject to drafts and- /or cooling devices, it is possible that a prolonged prestabilization period will not be necessary each time the lamp is turned on. However. if the lamp moves or is subject to cooling, in such a manner that the mercury is displaced from the position it occupies during stabilized operation, the lamp might require a prolonged pre-stabilization period each time it is turned on.

The presence of heat shields is believed to be responsible for this phenomenon due to the fact that they impede the circulation of gases to the extremities of the lamp and thereby make it difficult for the mercury vapor to come in contact with the coolest regions of the glass envelope of the lamp.

SUMMARY OF THE INVENTION In view of the foregoing, it is an object of the present invention to provide an improved low-pressure mercury vapor discharge lamp.

A principal object of the invention is to reduce the amount of time required for a heavily loaded fluorescent lamp to reach a state of stabilized light output.

These and other objects, advantages and features are attained, in accordance with the principles of this invention, by disposing a quantity of amalgam-forming material on a'surface within the lamp proximate the heat shield in the region between the heat shield and its respective end of the lamp. In this manner, the present invention remedies the above-described stabilization problem by providing means for retaining most of the mercury between the heat shield and the end of the lamp, even when the lamp is not in operation but is subjected to motion or cooling conditions that would normally displace the mercury from the position it occupies during stabilized lamp operation.

Heretofore, amalgam-forming materials have been used in fluorescent lamps to control the mercury vapor pressure of operating lamps, and thus optimize light output. An undesirable feature of such a use of amalgam-forming material is that in non-operating lamps, the mercury vapor pressure is so low that starting becomes difficult. As a result, secondary deposits of an amalgam-forming metal, such as indium, are placed at various hot spot locations to serve as a starting aid.

The present invention differs from such prior art teachings relative to amalgam lamps in the the amalgam-forming material is employed to store mercury, while the lamp is inoperative, in such a location that when the lamp is ignited, the mercury will be released rapidly into the cool end chamber where the operating mercury vapor. pressure will be controlled by the envelope wall temperature.

BRIEF DESCRIPTION OF THE DRAWINGS This invention will be more fully described hereinafter in conjunction with the accompanying drawings in which:

FIG. 1 is an elevational view, partly in section, of a fluorescent lamp embodying the present invention, a portion of the bulb being removed for convenience;

FIG. 2 is a cross-sectional view taken along line 22 of FIG. 1: and

FIG. 3 shows comparative curves of relative light output vs. time.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. I, a fluorescent lamp is shown comprising an elongated tubular glass envelope 10 having the customary coating II of phosphor on its inner surface and electrode mounts I2 sealed into each of its ends. The lighttransmitting envelope is filled with a small amount of rare gas, such as argon, at low pressure, e.g., one to three-Torr, and a small quantity of mercury, say 5-75 mgs., afterswhieh it is hermetically sealed inthe usual manner by tipping off an exhaust tube (not shown) at one or both ends of the lamp.

' Each mount sturcture 12 includes the typical reenthe usual alkaline-earth oxide (electron-emissive) coating and is supported on a pair of lead wires 18 and 19 sealed through the stem press 16. The lead wires extend to terminal pins 20 and 21 insulatively mounted in the lamp bases 22 attached to each end of the hermetically sealed, light-transmitting envelope 10.

Referring also to FIG. 2, the lamp further includes a heat shield 23 disposed at some point in the envelope between the cathode coil electrode 17 and its respective end of the lamp. More specifically, as illustrated, the heat shield 23 may comprise a thin circular metal plate attached, such as by a spot weld 24 to lead wire 18 and supported transversely in the envelope between the coil 17 and stem press 16. The other lead wire 19 passes through a hole in the heat shield plate 23 and is electrically isolated from the metal heat shield by an insulator sleeve 25.

of course, the heat shield may be made of a nonconducting material, such as glass or ceramics, and be fused to both lead wires. ln the present application,

however, metal is the preferred material for the heat shield, particularly nickel or nickel-plated iron. Other suitable metals are stainless steel, iron, steel, aluminum or titanium. Further, the heat shield may be clamped to the glass stem 14, rather than being attached to a lead wire.

The purpose of the heat shield is to retard the flow of heat (in an operating lamp) from the hot coil 17 and the hot plasma between electrodes to the cooler gas filled region and envelope wall that are between the heat shield and the base of the lamp. Thus, under operating conditions, the envelope wall between electrodes will be hotter than the envelope wall between the heat shield and the base, and the mercury vapor pressure in the lamp will be determined by the temperature of the posed on a surface within the lamp proximate the heat shield 23 in the region between the heat shield and its respective end of the lamp i.e., the indium is placed on one'or more hot spots in the cool end chamber. For example, possible locations would be on the inner leads, the stem, the envelope wall between the heat shield and base'but close to the heat shield. or a combination of these locations. As illustrated in FIGS. 1 and 2, however. a preferred location is to coat the amalgamforming material 26 on the heat shield 23 itself. on the side thereof facing away from the coil 17 and toward its respective end of the lamp; ln such case, a metal heat shield is preferred-as the amalgam-forming materialadheres better to metal than to glass or ceramic.

When the lamp is not in operation. most of the mercury will be absorbed and retained by the amalgamforming material 26 and thereby prevented from migrating to the central region of the lamp. When the lamp is switched on, the heat shield 23 heats up quickly due to its proximity to thev coil 17, and the mercury is driven from the amalgam directly into the cool chamber at the end of the lamp. It should be noted that the amalgam-forming material does not have to be used at both ends of the lamp, even if both ends have a heat shield.

According to one specific embodiment, a nickel heat shield 23 coated with 45 mgs. of indium (on the side away from the coil) is employed at one end ofa 22 inch VHO fluorescent lamp designed for operation at 25 watts per foot by using a T12 envelope (1.500 inches o.d.) and having a neon/30% argon fill at 2.5-2.9 Torr and containing 5 mgs. of mercury. Other amalgam-forming materials such as cadmium, may be employed, and the total amount of amalgam forming material disposed in the lamp may have a weight ratio to the mercury in the lamp of from about 1:4 to 30:l, although the preferred ratio is approximately 9:1.

A specific test result is shown by Curves A and B in FIG. 3. Curve A is a plot of relative light output versus burning time for a conventional VHO lamp, i.e., of the type described above but not containing the indium 26. The time from turn-on at t to stabilization at I is variable for different samples but may ,take as long as 24 hours. Curve B was obtained from the same lamp type as A except that lamp B contained 45 mgs. of indium attached to the heat shield in accordance with the invention. Lamp B required only 7 minutes from turn-on at 1,, to stabilization at r,,.

Curve C shows that the path to stabilization for lamps containing amalgam-forming material according to the invention is not necessarily the same as shown in B but may go through a peak. Whether a particular lamp follows a B or C type path will be determined .by the temperature of the cold region of the bulb during lamp operation. If the cold spot temperature is 40C or less. it will follow a B type path. If above 40C. it will follow a C type path.

ln summary, we employ an amalgam-forming material on hot spots between the heat shields and the bases of a fluorescent lamp for the purpose of absorbing mercury when the lamp is turned off and holding it in a location that is relatively close to that part of the lamp that controls light output during lamp operation. Such a location also permits fast heat-up and mercury release when thelamp is turned on. As a result, stabilization time is reduced. The mercury vapor pressure during lamp operation is controlled by the lamp cool end chamber and not by the amalgam spot. During long periods of lamp storage the mercury will remain behind the heat shield and within the cool end chamber. During normal lamp shipment and under normal vibration stresses caused by lamp shipment, the mercury will remain physically located within the cool end chamber. The deposition of either excess and/or free mercury on the bulb wall, which causes irregular light output profiles that are quite undesirable for reprographic applications. will be minimized. And'the placement and control of mercury for those lamps manufactured for the reprographic'industry becomes less critical.

Although the invention has been described with respect to specific embodiments. it will be appreciated that modificationsand changes may be made by those skilled in the art without departing from the true spirit and scope of the invention. Accordingly, the invention is not limited to the described T12 lamps having a loading of 1.500 ma. but can also be advantageously useful in, for example, T5 lamps having a loading of 250 ma. or greater, T8 lamps having a loading of 550 ma. or greater, Tl2 lamps having a loading of l,000 ma. or greater, and T 17 lamps having a loading of 1,500 ma. or greater.

What we claim is:

l. A low-pressure mercury vapor discharge lamp comprising:

an hermetically sealed, elongated light-transmitting envelope containing an inert ionizable fill gas and a quantity of mercury,

a mount structure including a stem sealed to and extending inwardly from each end of said envelope and an electrode supported at the inward end of each of said stems,

at least one of said mount structures having attached thereto a fixed heat shield disposed between the electrode thereof a fixed heat shield disposed between the electrode thereof and its respective end of said lamp for providing a relatively cool end chamber in said lamp during the operation thereof,

and

a quantity of amalgam-forming material disposed on a surface within said lamp proximate said heat shield and solely in the region between said heat shield and its respective end of said lamp, whereby said amalgam-forming material is operative to absorb and store mercury in said region while the lamp is inoperative and rapidly release the mercury into said region when the lamp is ignited, whereupon the mercury vapor pressure is controlled by said cool end chamber and the time required to reach a state of stabilized light output is reduced.

2. A lamp according to claim 1 wherein said amalgam-forming material is disposed on said heat shield.

3. A lamp according to claim 1 wherein said heat shield is a thin circular plate supported transversely in said envelope.

4. A lamp according to claim 3 wherein said amalgam-forming material is coated on the side of said heat shield plate facing its respective end of said lamp.

5. A lamp according to claim 1 wherein said heat shield is made of nickel, nickel-plated iron, stainless steel, iron, steel, aluminum or titanium.

6. A lamp according to claim 5 wherein said amalgam-forming material is indium.

7. A lamp according to claim 1 wherein said electrode comprises a cathode coil supported on a pair of lead wires sealed through said stem.

8. A lamp according to claim 8 wherein said heat shield comprises a thin circular metal plate attached to one of said pair of lead wires and supported transversely in said envelope, the other of said lead wires passing through a hole in said heat shield plate, and further including means insulating said other of the lead wires from said metal heat shield plate.

9. A lamp according to claim 1 wherein said lamp is a very high output fluorescent lamp adapted for a power loading of at least about 25 watts per foot, and the total amount of said amalgam-forming material disposed in said lamp has a weight ratio to the mercury in said lamp of about 9:1. 

1. A low-pressure mercury vapor discharge lamp comprising: an hermetically sealed, elongated light-transmitting envelope containing an inert ionizable fill gas and a quantity of mercury, a mount structure including a stem sealed to and extending inwardly from each end of said envelope and an electrode supported at the inward end of each of said stems, at least one of said mount structures having attached thereto a fixed heat shield disposed between the electrode thereof a fixed heat shield disposed between the electrode thereof and its respective end of saId lamp for providing a relatively cool end chamber in said lamp during the operation thereof, and a quantity of amalgam-forming material disposed on a surface within said lamp proximate said heat shield and solely in the region between said heat shield and its respective end of said lamp, whereby said amalgam-forming material is operative to absorb and store mercury in said region while the lamp is inoperative and rapidly release the mercury into said region when the lamp is ignited, whereupon the mercury vapor pressure is controlled by said cool end chamber and the time required to reach a state of stabilized light output is reduced.
 2. A lamp according to claim 1 wherein said amalgam-forming material is disposed on said heat shield.
 3. A lamp according to claim 1 wherein said heat shield is a thin circular plate supported transversely in said envelope.
 4. A lamp according to claim 3 wherein said amalgam-forming material is coated on the side of said heat shield plate facing its respective end of said lamp.
 5. A lamp according to claim 1 wherein said heat shield is made of nickel, nickel-plated iron, stainless steel, iron, steel, aluminum or titanium.
 6. A lamp according to claim 5 wherein said amalgam-forming material is indium.
 7. A lamp according to claim 1 wherein said electrode comprises a cathode coil supported on a pair of lead wires sealed through said stem.
 8. A lamp according to claim 8 wherein said heat shield comprises a thin circular metal plate attached to one of said pair of lead wires and supported transversely in said envelope, the other of said lead wires passing through a hole in said heat shield plate, and further including means insulating said other of the lead wires from said metal heat shield plate.
 9. A lamp according to claim 1 wherein said lamp is a very high output fluorescent lamp adapted for a power loading of at least about 25 watts per foot, and the total amount of said amalgam-forming material disposed in said lamp has a weight ratio to the mercury in said lamp of about 9:1. 